In one method of exposing a photographic photosensitive material, an image is formed using a so-called scanner system in which an original is scanned and a silver halide photographic material is exposed on the basis of an image signal, and an image is formed as a negative image or as a positive image corresponding to the image on the original.
There are various recording devices in which image formation via a scanner system is used in practice, and conventionally glow lamps, xenon lamps, mercury vapor lamps, tungsten lamps and light emitting diodes, for example, have been used as the light sources for recording purposes in scanner type recording systems of this type. However, these light sources all suffer from the disadvantages of weak output and short life expectancy in practical use. Scanners in which coherent laser light sources, such as Ne-He lasers, argon lasers and He-Cd lasers for example, are used as light sources have been proposed as a means of overcoming these problems. It is possible to obtain high outputs in this way, but the apparatus is larger and more expensive and a modulator is required, and there is a further disadvantage in that safe-lighting for the photosensitive material is limited since visible light is used for exposure, and handlability is adversely affected.
On the other hand, semiconductor lasers are small, cheap and easily modulated, and they have a longer life expectancy than the lasers mentioned above. Moreover, if a photosensitive material which is sensitive to the infrared region is used, then since the light emission is in this region, there is a distinct advantage in that bright safelights can be used and handlability is greatly improved. However, there is no photosensitive material which has a high sensitivity in the infrared region and excellent storage properties and so it has not been possible to realize the characteristics of the semiconductor lasers which provide superior performance as described above.
An "HIE135-20" film made by Eastman Kodak Company is an example of a commercially available infrared sensitized film, but this material cannot be stored at room temperature and it must be stored in a refrigerator or freezer. Thus, the conventional infrared sensitized photosensitive materials are unstable in respect of photographic speed, and special consideration must be given to their storage.
In one technique for the manufacture of photographic photosensitive materials, the photosensitive wavelength region is extended to the long wavelength side by adding certain types of cyanine dye to the silver halide photographic emulsion, which is to say that spectral sensitization techniques are used, and these spectral sensitization techniques can be applied not only to the visible region but also to the infrared region. Sensitizing dyes which absorb infrared light are used for infrared region spectral sensitization, and such dyes have been described, for example, by Mees, The Theory of the Photographic Process, Third Edition, pages 198-201 (published by MacMillan, 1966). In this case the spectral sensitivity, which is to say the sensitivity to light in the infrared region, should be high, and any change in photographic speed on storing the emulsion should be slight. A number of sensitizing dyes have been developed for this purpose in the past.
Thus, dyes of this type have been disclosed, for example, in U.S. Pat. Nos. 2,095,854, 2,095,856, 2,955,939, 3,482,978, 3,552,974, 3,573,921 and 3,582,344. However, it cannot be said that the photographic speed and storage properties are adequate when the sensitizing dyes disclosed in these patents are used.
On the other hand, spectral sensitivity can be increased markedly by adding certain types of specially selected second organic compounds to photosensitive materials in addition to the spectral sensitizing dyes, and the resulting effect is known as supersensitization. In general, the addition of a second organic compound or an inorganic compound does not increase the sensitivity, and it may even lower the sensitivity in many cases, and so supersensitization can be said to be a special phenomenon and very careful selection is required for the sensitizing dyes and second organic compounds or inorganic compounds which are used in such combinations. Moreover, it is thought that slight differences in chemical structure have a pronounced effect on this supersensitizing action and it is difficult to predict supersensitizing combinations on the basis of chemical structure alone. Second organic compounds for supersensitization purposes known at present time include, for example, the triazine derivatives disclosed in U.S. Pat. Nos. 2,875,058 and 3,695,888, the mercapto compounds disclosed in U.S. Pat. No. 3,457,078, the thiourea compounds disclosed in U.S. Pat. No. 3,458,318, and the pyrimidine derivatives disclosed in U.S. Pat. No. 3,615,632, and it has also been disclosed that infrared sensitization can be achieved using smaller amounts of infrared sensitizing dyes by using the azaindene compounds disclosed in U.S. Pat. No. 4,011,083.
The infrared photographic speed is certainly increased with the methods disclosed in these patents, and fairly good storage properties have also been observed, depending on the particular case, but as yet they are still inadequate, and a means of supersensitization which provides a large increase in infrared photographic speed and good storage properties is still required.